1. Field of the Invention
The present invention relates to a moving object detection apparatus for detecting a moving object and, more particularly, for detecting whether an object, such as a human being, a vehicle, an obstruction, a fixed object, or the like (hereinafter collectively referred to as an “object”), is moving or not.
2. Description of the Related Art
FIG. 1 shows a block diagram of a moving object detection apparatus 1. An output of an oscillator circuit 11 is radiated from a transmitting antenna 12 toward an object, and the wave reflected by the object is received by a receiving antenna 13; the received wave is then mixed in a receiver circuit 14 with a portion of the transmitted wave to produce a beat signal. This beat signal, as a detection output, is passed on to a detector circuit which outputs a moving object detection signal.
If the object is moving, the reflected wave is shifted in frequency due to the Doppler effect. When the transmitted frequency is f0, for example, the reflected frequency is f0+Δ. Here, the amount of shift, Δ, is derived from the following equation.Δ=Reflected frequency−Transmitted frequency=(2v/c)f0  (1)
v: Relative velocity of moving object with respect to detection apparatus
c: Velocity of light
FIG. 2 is a diagram illustrating the principle of a method for detecting a moving object using the above moving object detection apparatus. In FIG. 2, x0 indicates the position of the moving object 2 at t=0; when the transmitted wave Asin(ω0t) radiated from the transmitting antenna 12 is reflected by the moving object 2 approaching at a velocity v, the reflected wave is given as B sin {ω0(t−2x/c)}. Here, x is the position of the moving object 2, and c is the velocity of light. If x=x0−vt, then. the reflected wave is given asB sin [ω0 {t−2(x0/c−vt/c)}]B sin [ω0(1+2v/c)t−ω0(2x0/c)}  (2)
FIG. 3 is a diagram showing the phase relationship between the wave transmitted from the moving object detection apparatus 1 and the wave reflected from the object 2. As shown in FIG. 3, when the object is stationary, the phase difference between the transmitted wave and the reflected wave is a function of the distance L between the object and the moving object detection apparatus 1. In FIG. 3, (a) shows the waveform of the transmitted wave, (b) the waveform of the reflected wave with a phase difference of 0, and (c) the waveform of the reflected wave with a phase difference of 90°. The phase difference between the transmitted wave and the received wave depends on the distance between the moving object detection apparatus 1 and the object 2.
FIGS. 4A to 4C are diagrams each showing the relationship between the transmitted wave (a) and the reflected wave and their superimposed wave. FIG. 4A shows the superimposed wave (Sab) when the phase difference θ of the reflected wave is 0. FIG. 4B shows the superimposed wave (Sac) when the phase difference θ of the reflected wave (c) is 90°. FIG. 4C shows the superimposed wave (Sad) when the phase difference θ of the reflected wave (d) is 180°.
As can be seen from FIGS. 4A to 4C, the waveform of the superimposed wave produced by combining the transmitted wave with the received wave varies with the distance between the object and the detection apparatus, provided that the size of the object and the material of its surface remain the same. Therefore, when the object is moving, the superimposed wave (S) varies as shown in FIG. 5. When the amount of change of the envelope (E) of the superimposed wave (S) is extracted, a signal proportional to the relative velocity of the object can be obtained, thus making it possible to detect whether the object is moving or not.
FIG. 6 is a diagram showing a one-element type moving object detection apparatus according to the prior art. As shown in FIG. 6, an oscillator 11, a transmitting antenna 12, a receiving antenna 13, etc. are placed within a cavity resonator 16, and a moving object detection signal is obtained by transmitting and receiving waves through an opening 16-1 formed in a portion of the cavity resonator 16.
FIG. 7 is a diagram showing a two-element type moving object detection apparatus according to the prior art. As shown in FIG. 7, an oscillator 11, a transmitting antenna 12, a receiving antenna 13, a coupler 15, a diode D as a detector, etc. are placed within a cavity resonator 16, and a moving object detection signal is obtained by transmitting and receiving waves through an opening 16-1 formed in a portion of the cavity resonator 16.
An FET such as a GaAs FET or a heterojunction FET (HEMT) is used as the oscillator.
As shown, each of the prior art moving object detection apparatuses is provided with two separate antennas, the transmitting antenna 12 and the receiving antenna 13, and a beat component is extracted as the detection output from the drain or source of the FET functioning as the oscillator. All the circuit components are mounted on a board, and the entire structure is housed in the cavity resonator 16, with the antennas located near the opening formed in a portion of the cavity resonator; if needed, an external antenna can be connected.
FIG. 8 is a diagram schematically showing the arrangement of the cavity resonator 16, the circuit board 18 with the circuit components such as the transmitting antenna 12 and the receiving antenna 13 mounted thereon, and the external antenna 17. The circuit board 18 is housed in the cavity resonator 16 for enhanced antenna efficiency, etc., and the opening 16-1 is formed in the cavity resonator 16 so that the antennas 12 and 13 can transmit and receive waves to and from the outside. The external antenna 17, if needed, is mounted in the opening 16-1 formed in the cavity resonator 16.
Of the above prior art moving object detection apparatuses, the one-element type shown in FIG. 6 is simple in circuitry, but has the problem that satisfactory reception sensitivity cannot be obtained if the amplification gain of the FET is not large enough.
On the other hand, the two-element type shown in FIG. 7 has the problem that the cost increases because of the increased circuit complexity and the increased number of elements.
In both the one-element type and the two-element type, as all the circuit components are housed in the cavity resonator, an antenna has to be connected to the opening when changing the moving object detection area, and, hence, there is the problem that not only the cost but also the size of the detector increases.
Accordingly, it is an object of the present invention to provide a moving object detection apparatus that is simple in configuration and that achieves reductions in size and cost.